We have studied the effects of various protein modifying chemicals on the ACh-receptors of Aplysia ganglion cells and have compared the effects on both the excitatory (D-type) and inhibitory (H-type) receptor activities. Each reagent was mixed with Aplysia Ringer and directly applied to the identified cells by constant speed perfusion. 1) Pyridoxal 5'-phosphate (PLP) was found to block H-type but not D-type responses of ACh-receptor membranes. ACh-induced H-type responses were gradually depressed by exposure of the receptor membrane to 1 mM PLP. At pH 8.0, ACh-induced increase in membrane conductance (delta G) of the H-type decreased to 20-30 percent of the control within 20-25 min. of exposure to 1 mM PLP. Two to three hours of continuous washing with the normal Ringer gradually restored the normal response to ACh, however. The ACh-induced D-type response was not appreciably altered even after 40 min. exposure. 2) p-Nitrothiophenol (NTP) was found to block D-type but not H-type responses of ACh-receptor membranes. ACh-induced D-type responses were irreversibly depressed after a brief exposure to 1 mM NTP at pH 7.0. A five min. exposure decreased delta G to 60-70 percent of the control and an additional 10 min. exposure reduced delta G to 30-40 percent of the control. The log(ACh) - delta G curves showed a systematic shift, i.e., decrease in slope, with increase in exposure time. ACh-induced H-type response was not appreciably altered even after 60 min. exposure to 1 mM NTP. 3) Mersalyl (Na-salt of Salyrganic acid) was found to block H-type but not D-type responses of ACh-receptor membranes. The ACh-induced delta G of H-type was markedly depressed to 20 percent of the control after a 5 min. exposure to 1 mM Mersalyl at pH 7.5. The log(ACh) - delta G curve was shifted to the right with a marked decrease in the slope during the depression. The response, however, gradually recovered over a period of 2-3 hours after continuous washing with normal Ringer. The ACh-induced delta G of D-type was not significantly altered by a 5-10 min. exposure to 1 mM Mersalyl.